* move elementwise_div grad

* change mutable_data to alloc

* fix compile bugs

* move reduce gpu impl funcs into pten/kernels/funcs

* change reduce header name and namespace

* fix spell word error

* change mutable_data to dev_ctx.Alloc

* modify place to devcontex

* format code style

* fix build error

* fix build error

* fix conflict

* move elementwise function to funcs directory

* fix compile bugs

* modify according to comment